How to Office 365 smtp relay server

Published October 10, 2024 · Updated July 18, 2026 · SMTP & Deliverability

The Office 365 SMTP relay server lets your organisation's devices and applications send email through Microsoft 365. It is a dependable option for internal and business mail - within Microsoft's limits. Here is how the Office 365 relay server works and where it fits.

How the relay server works

Depending on your setup, mail routes through Microsoft 365 via SMTP AUTH (as a mailbox), direct send, or a connector authorised by static IP. Each path uses Microsoft's servers and inherits its recipient limits.

Key setup points

Use smtp.office365.com port 587 for authenticated submission.
Enable SMTP AUTH on the mailbox (off by default on new tenants).
For a connector, authorise your static sending IP and add it to SPF.

The limit: the Office 365 relay server is built for business mail and caps recipients per day. For marketing volume, route campaigns through a dedicated SMTP.

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Frequently asked questions

What is the Office 365 SMTP relay server?

Microsoft 365's service for relaying device and app mail via SMTP AUTH, direct send, or a connector - using smtp.office365.com and your MX endpoint.

Can I send bulk email through it?

No - it caps recipients per day and is built for business mail. Use a dedicated SMTP for bulk campaigns.